Titanium dioxide is produced as a pigment either by reaction of TiCl.sub.4 with O.sub.2 or by hydrolysis of an aqueous solution of titanyl sulfate containing sulfuric acid and subsequent calcination.
In the case of the sulfate process, the titanium dioxide hydrate suspension is calcined for 5 to 20 hours in a rotary kiln. The deposit which accumulates during hydrolysis consists of fine particles having a specific BET surface of 50 to 500 m.sup.2 /g which undergo extensive agglomeration during calcination.
The agglomerates formed as a result of calcination are in the form of 0.1 to 1.0 mm diameter particles which are made up of 0.2 to 0.4 .mu.m primary particles. This material is unsuitable as clinker or even as ground pigment for incorporation in vitreous enamels or fusion enamels because it agglomerates when mixed in dry form with the other constituents of the enamel frit, resulting in incomplete and inhomogeneous dissolution of the titanium dioxide during melting of the frit.
DE-AS No. 1 207 363 describes a process for the production of a TiO.sub.2 product for incorporation in vitreous enamels by calcination of precipitated titanium dioxide hydrate. In this process, a precipitated titanium dioxide hydrate of coarse particle size, which consists of 5 to 20 .mu.m primary particles, is mixed with a precipitated titanium dioxide hydrate of fine particle size, which consists of primary particles mostly below 1 .mu.m in size, as obtained in the production of TiO.sub.2 pigments, and the resulting mixture is calcined. The disadvantage of this process is that it is extremely expensive on account of the two different titanium dioxide hydrates which first have to be produced. In addition, the disadvantage of TiO.sub.2 obtained in this way is that it is distinguished by a high apparent density of 1.4 to 2 kg/l and has a low specific surface of distinctly below 1 m.sup.2 /g, resulting in inadequate reactivity during melting in the frit.
U.S. Pat. No. 2,721,787 describes free-flowing, dust-free aggregates consisting of calcined anatase (primary particle size 0.2 to 0.3 .mu.m, specific surface 4.5 to 9 m.sup.2 /g) and 0.1 to 3% of an inorganic binder. These aggregates are produced by spray-drying aqueous slurries of hydro-classified titanium dioxide pigments emanating from normal pigment production at a temperature in the range from 150.degree. to 400.degree. C. The disadvantage of this process is that calcined titanium dioxide pigments first have to be produced and are then taken up in water to form a slurry which is then spray-dried to obtain the claimed aggregates.
U.S. Pat. No. 3,018,186 describes a platelet-like titanium dioxide having a diameter of 1 to 20 .mu.m and a thickness of 0.01 to 0.5 .mu.m which is almost exclusively present in the rutile modification. These platelets are produced by calcining a titanyl sulfate solution containing at least 5% and at most 20% of sulfuric acid in a flame for 0.001 to 0.5 seconds at 800.degree. to 1800.degree. C. The material obtained is free from sulfuric acid and is used in ceramic and electrical materials, plastics and surface coatings and also as a filler. The titanyl sulfate solution used is expensive to prepare because, in pure form, it does not represent an intermediate product of the pigment production process. The solution formed by digestion of titanium ores contains considerable quantities of foreign elements, such as Fe, Al, Mg, etc.